Effective usage of the energy is realized with the use of an electricity storage device such as a secondary battery. For example, in recent years, solar light power generation systems have been actively developed as eco-friendly, clean energy sources. Because a solar cell module which converts the solar light into electric power does not have an electricity storage function, in some cases the solar cell module is used in combination with a secondary battery. For example, the energy is effectively used by charge and discharge control to charge the electric power generated by the solar cell module into the secondary battery and to discharge the electric power from the secondary battery in response to a request from an external load or the like.
As a technique related to the present invention, for example, Patent Literature 1 discloses a power supply device of a solar cell having a solar cell, a plurality of secondary batteries which are charged by the solar cell, a charge switch which is connected between each secondary battery and the solar cell and which controls charging of the secondary battery, a discharge switch which is connected between each secondary battery and a load, and a control circuit which controls the charge switch and the discharge switch. In this reference, the control circuit identifies the order of priority of the secondary batteries to be charged by controlling the plurality of charge switches, a secondary battery with a higher priority order is charged before a secondary battery with a lower priority order, and, after the secondary battery with the higher priority order is charged to a predetermined capacity, the secondary battery with the lower priority order is charged.